Whenever I view some form, it doesn't matter what kind of form, and edit some values, save it and then view some similar form, there will be data from the previous form in the fields, which almost renders Webmin useless. I've confirmed this with Firefox 4.0, after a quick test IE7 seems to be doing fine. All cookie setting seem to be right.
I'm not sure why the bug occurs, and maybe it's not Webmin's fault but Firefox's. But since Firefox is probably the main browser used for viewing Webmin, it seemed to me it's kind of important to report this bug.
That sounds like a firefox bug or mis-configuration to me - it is auto-filling in forms for you. I strongly doubt if this this is a bug in Webmin.